DESCRIPTION: (Adapted from the applicant?s description) Fragile X syndrome is caused by molecular defects in the structure and expression pattern of the FMR1 gene on the X chromosome. It is fairly well accepted that CGG repeat expansion leads to DNA methylation which then inhibits FMR1 activity, but very little is known about how this occurs. In order to decipher this mechanism, the investigators will use transgenic mice and embryonic cell culture systems. Studies on the cis acting elements and trans acting factors which regulate normal DNA methylation in the early embryo will form the basis for investigating the aberrant factors which bring about faulty methylation at the FMR1 locus. Studies on chromatin structure will be aimed at explaining how DNA methylation actually causes transcriptional inhibition. Another structural parameter which is affected in fragile X syndrome is DNA replication timing. Using FISH technology, the investigators plan to elucidate the mechanism which causes the FMR1 region to replicate so late in the cell cycle, and thus understand how this brings about regional repression. Unlike other genetic diseases, the molecular changes in fragile X syndrome are epigenetic in nature, so that once these mechanisms have been better defined, it should be possible to design new strategies for reactivating FMR1, thereby reversing the molecular defect.